`UseEnum` does not work with str enum

Slightly adapting the `UseEnum` example to use a `str` based enum makes it fail to recognize the enum by the value:

```
import enum
from traitlets import HasTraits, UseEnum

class Color(str, enum.Enum):
RED = "red"
BLUE = "blue"
GREEN = "green"

class MyEntity(HasTraits):
color = UseEnum(Color, default_value=Color.BLUE)

entity = MyEntity(color=Color.RED)
entity.color = Color.GREEN
entity.color = "GREEN"
entity.color = "Color.GREEN"
entity.color = "green"
assert entity.color is Color.GREEN
```

Raises:

```
❯ python test_enum.py
Traceback (most recent call last):
File "/home/mnoethe/Projects/test_enum.py", line 16, in
entity.color = "green" # USE: number (as int)
File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 729, in __set__
self.set(obj, value)
File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 703, in set
new_value = self._validate(obj, value)
File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 735, in _validate
value = self.validate(obj, value)
File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 3582, in validate
self.error(obj, value)
File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 841, in error
raise TraitError(e)
traitlets.traitlets.TraitError: The 'color' trait of a MyEntity instance expected any of ['RED', 'BLUE', 'GREEN'], not the str 'green'.
```